This invention relates to phase-staggered power converters connected in parallel to a load, and more particularly to apparatus for sensing the failure of an active converter module and automatically substituting a standby converter module in proper phase sequence.
Electrical power systems used in satellites, and other long-life electrical apparatus of a similar nature employ converters for conditioning the output of a DC power source. For satellites the power source is typically comprised of solar cell arrays, but for other applications, such as deep ocean buoys, the power source may be battery cells.
Reliability of operation throughout the expected life of the apparatus, such as the life of a mission of a satellite, is essential. This can be provided insofar as the power supply is concerned by utilizing small converter modules operating in parallel. An attractive feature of this approach is that new systems can be tailored to new requirements by simply changing the number of modules without any need for developing new circuits.
However, because of the high voltage input of typically 400 volts, design of an input filter is a challenge since conventionally used filter capacitors are of the tantalum electrolytic type which are not reliable at such high voltages. The type of capacitor best suited for such applications is the metallized polycarbonate dielectric type, but such capacitors are available only in very slow capacitance valves. By using a staggered phase approach, it is possible to increase the equivalent input current ripple frequency by a factor of four while simultaneously decreasing the ripple amplitude of the current required from the input filter for the power stages. This makes it possible to use much smaller capacitors in the input filter.
For the necessary reliability, one approach has been the provision of simple redundancy. A spare converter module is included with provision to switch to the spare in the event one of the converters on the line fails. The concept of substituting a standby power unit for one that has failed has been applied in many different arrangements representative of which are systems disclosed in the following U.S. Pat. Nos.: 2,546,410; 2,694,155; 3,193,705; 3,505,531 and 3,811,050. However, the power units in these systems are independent and do not involve phase-staggered units connected in parallel to a load such that substitution of a standby unit presents no real problem once the failure of a unit is detected. In the earliest U.S. Pat. No. 2,546,410, the system disclosed is for substituting a spare turbo-generator for a failed turbo-generator. There the independent units deliver power to buses at different frequencies so that it is necessary to adjust the frequency of the spare to the frequency of the unit for which the spare is being substituted. However, that is a distinct problem from substituting a spare in a multiphased system of modules delivering power to a common load.
The advantages of phase-staggered converter modules go beyond increasing reliability and the ability to use much smaller capacitors in the input filter. When a power converter fails in a system which does not phase-stagger the converters, there is an interruption of power while the spare is being switched. This interruption, even if only of short duration, creates surges and transients which adversely affect operation of the load. This is of significance to space missions where the power demands continue to increase with each successive mission. Also, space missions of longer duration have increased the need for greater reliability. Interruption of power during the switching period is significantly reduced by operating the converter modules supplying power to a common load in a multiphase or staggered sequence. However, since failure is unpredictable, i.e., random, substitution of the standby converter in proper phase presents a problem. If not substituted in proper phase, so that the multi-phase arrangement is not disturbed, the aforementioned advantages of the system are lost in that surges and transients are created that may adversely affect operation of the various electronic equipments of the space mission, and which may require larger filter capacitors.